And the Dams Came Down: Deconstructing dams and myxozoan disease following Klamath River Dam Removals

Four large dams were removed from the Klamath River in Oregon and California, USA in 2023-24. This was the largest dam removal and restoration project in U.S. history, with a primary aim to restore critical habitat for threatened and endangered salmon populations that the dams had constrained to the lower half of the river basin for over a century. The dam removals resulted in profound physical changes to the river that are expected to alter host-pathogen dynamics in significant ways. Reconnecting the lower and upper portions of the river basin has increased habitat available to salmon, and adult salmon have already begun to return to streams upstream from the former dams where they were historically present. These fish now have longer migration routes, altering the distribution and abundance of both hosts and pathogens. To develop a management plan for the reconnected river, our workgroup drew from a combination of Western Science and Traditional Ecological Knowledge to inform disease risk predictions. While our initial synthesis included multiple pathogens for which there was evidence of disease related impacts, in this forum we focus on Ceratonova shasta , which is considered the primary pathogen limiting salmon recovery in this river system. For juvenile salmon, we predict a reduction in disease risk resulting from the decrease in overlap between infected salmonid and annelid hosts, attributable to the restoration habitats that favor salmonids but are less suitable for annelids. For both juvenile and adult salmon that utilize the newly accessible upper river habitats, we predict increased risk of infection and disease resulting from extended migration routes. For adults, the increased risk is expected to be associated with prespawn mortality, previously not observed in salmon in this river system. Long term, we expect the combination of increased diversity in salmonid life histories in combination with the availability of more and diverse habitat will temper disease risk and restore some balance to this host-parasite cycle.